Abstract

The prefrontal cortex (PFC) has a well-established role in the inhibition of inappropriate responding, and evidence suggests that the infralimbic (IL) region of the rat medial PFC (MPFC) may be involved in some aspects of extinction of conditioned fear. MPFC lesions including, but not those sparing the IL cortex increase spontaneous recovery of extinguished conditioned fear when tested 24 h after an initial extinction session. The current experiment extended these findings by use of appetitive rather than aversive conditioning. Ten IL-lesioned and 11 sham-operated rats were trained on a Pavlovian task in which a conditioned stimulus (CS) was followed by food pellets (the unconditioned stimulus or US). IL lesions had no effect on extinction of the conditioned response (CR, magazine entries) during the first extinction session. However, the level of spontaneous recovery between the first extinction session and a second, 24 h later, was increased in IL-lesioned rats relative to sham animals. In contrast, evidence of savings measured between the extinction sessions did not differ between groups. Furthermore, reinstatement of the CR following unsignaled delivery of the US was also increased in IL-lesioned rats.

Schematic representation of the IL lesion showing the maximum (dark gray) and minimum (light gray) extent of cell damage and gliosis due to ibotenic acid infusions. Reconstructions were made on the basis of coronal sections (Paxinos and Watson 1997) at appropriate distances (millimeter) anterior to bregma.

Mean CRs (mean CS-preCS magazine entries per minute ± SEM) across extinction sessions. Lines represent responding in blocks of four trials across the first extinction session and the recovery and reinstatement sessions. The first bar (Trial 1) represents responding on the first trial of the first extinction session. The second and third bars represent the mean responding on the first trial of extinction sessions testing for spontaneous recovery (Rec; i.e., sessions 2, 4, and 6) and reinstatement (Rein; i.e., sessions 3, 5, and 7), respectively.